1. Field of the Invention
The present invention relates to an electronic device having perovskite-type oxide film, a process for producing the same, and a ferroelectric capacitor.
2. Description of the Related Art
Prevailing computers, regardless of their size, have the main memory composed of dynamic random access memory (DRAM) or static random access memory (SRAM) which are of volatile memory. Volatile memory holds data while it is supplied with electric power but loses stored data as soon as the supply of electric power is suspended. By contrast, non-volatile memory holds data even without power supply. One of such non-volatile memory attracting attention is FeRAM which is based on ferroelectrics. The advantage of FeRAM is non-volatility, low power consumption, and capability of high integration density. In addition, because of its greatly improved rewritability, FeRAM is expected to supersede the existing memory and to find use in new applications such as IC card.
The constituent element of FeRAM is a ferroelectric capacitor, which is constructed of a ferroelectric film and two electrodes of platinum (Pt) or the like holding it between them. The ferroelectric film is required to have a high degree of residual polarization, a hysteresis loop with good squareness ratio and low coercive field, and a good fatigue characteristic (or ability to retain polarization even after repeated applications of pulses more than 10.sup.12 times). Its additional requirement is an ability to prevent leak current between capacitor electrodes when it forms a ferroelectric capacitor. Leak current aggravates hysteresis. Another requirement is good (low) imprint characteristics, which is an ability to keep the hysteresis unshifted when the ferroelectric capacitor is kept at a high temperature in its polarized state.
Among known ferroelectric materials suitable for ferroelectric capacitors is known lead titanate zirconate (Pb(Zr,Ti)O.sub.3) (abbreviated as PZT hereinafter). PZT is characterized by its comparatively high residual polarization even at room temperature and its sufficiently high Curie point compared to the operating temperature. When used for ferroelectric capacitor of layered structure (Pt/PZT/Pt), PZT suffers so-called fatigue (which is a phenomenon that residual polarization decreases when pulses are applied repeatedly more than 10.sup.6 times). This is detrimental to reliability.
An FeRAM less subject to fatigue is attracting attention. It is provided with conductive oxide electrodes, such as SrRuO.sub.3 (abbreviated as SRO), in place of platinum. These conductive oxide electrodes function as a barrier against diffusion of Pb and prevent the oxygen defects of ferroelectric materials. This is the reason whey they protect ferroelectric film from fatigue.
Ferroelectric capacitors with SRO electrodes have been reported by Argonne National Laboratory in USA and Texas Instruments Corp. (TI) in USA. The one reported by the former is not suitable for mass production because it uses SrTiO.sub.3 single crystal as the substrate. The one reported by the latter has PZT film deposited by sol-gel method on the SRO film deposited by high-temperature sputtering. Its disadvantage is that residual polarization is as small as 14.3 .mu.C/cm.sup.2 when the applied voltage is 3V. Another disadvantage is that high-temperature sputtering to deposit SRO film tends to cause oxygen defects (probably due to operation under reduced pressure), which poses a problem with crystallinity, although it yields polycrystalline film (in which crystals are oriented) if the substrate is heated during sputtering.
Furthermore, it is anticipated that it would be difficult to uniformly heat the entire surface of the substrate of large diameter and it would be difficult to form SRO film of good quality on the entire surface of the substrate of large diameter.
Although SRO is a conductive oxide, it has a resistance of 350 .mu..OMEGA.cm, which is slightly higher than that of Pt (11 .mu..OMEGA.cm). This makes it difficult to establish a good ohmic contact with Ti.sub.1-x Al.sub.x N which forms an adhesion layer when plugs are made of Si or W. Therefore, there is a demand for SRO having bottom resistance.